Method for stripping uranium(vi) and an actinide(iv) from an organic solution by oxalic precipitation

Abstract

A method for stripping U(VI) and an An(IV) from an organic solution including tri-n-butyl phosphate in an organic diluent, the solution containing U(VI) and the An(IV) present as U(VI) nitrate and An(IV) nitrate at concentrations such that the U(VI) nitrate concentration is higher than the An(IV) nitrate concentration, and the sum of the U(VI) nitrate and An(IV) nitrate concentrations is ≥55 g/L. The method includes contacting the organic solution and an aqueous solution of nitric and oxalic acids, the oxalic acid concentration in the aqueous solution and the O/A volume ratio selected so that the oxalic acid is deficient with respect to the stoichiometric conditions of a complete precipitation of U(VI) and actinide(IV), to obtain a precipitate containing the actinide(IV) in oxalate form and a fraction of the U(VI) in oxalate form with a U(VI)/actinide(IV) mass ratio of between 0.5 and 5; and separating the precipitate from the organic and aqueous solutions.

1 . A method for stripping uranium(VI) and an actinide(IV) from an organic solution in which the uranium(VI) and the actinide(IV) are present as uranium(VI) nitrate and actinide(IV) nitrate at concentrations such that a uranium(VI) nitrate concentration is higher than an actinide(IV) nitrate concentration, and a sum of the uranium(VI) nitrate concentration and the actinide(IV) nitrate concentration is greater than or equal to 55 g/L, the organic solution comprising tri-n-butyl phosphate in an organic diluent, the method comprising:
 contacting the organic solution and an aqueous solution comprising from 2 mol/L to 6 mol/L of nitric acid, and oxalic acid at a concentration greater than or equal to 18 g/L, with an organic solution/aqueous solution volume ratio greater than or equal to 1, wherein the oxalic acid concentration in the aqueous solution and the organic solution/aqueous solution volume ratio is selected so that the oxalic acid is deficient with respect to the stoichiometric conditions of a complete precipitation of uranium(VI) and actinide(IV), to obtain a precipitate comprising the actinide(IV) in oxalate form and a fraction of the uranium(VI) in oxalate form with a U(VI)/actinide(IV) mass ratio of between 0.5 and 5; and   separating the precipitate from the organic solution and aqueous solution.   
     
     
         2 . The method of  claim 1 , wherein the organic solution comprises from 25% to 35% (v/v) of tri-n-butyl phosphate. 
     
     
         3 . The method of  claim 2 , wherein the organic solution comprises 30% (v/v) of tri-n-butyl phosphate. 
     
     
         4 . The method of  claim 1 , wherein the oxalic acid concentration in the aqueous solution is greater than or equal to 20 g/L. 
     
     
         5 . The method of  claim 4 , wherein the oxalic acid concentration in the aqueous solution is greater than or equal to 22 g/L. 
     
     
         6 . The method of  claim 1 , wherein the organic solution/aqueous solution volume ratio is greater than or equal to 1.5. 
     
     
         7 . The method of  claim 1 , further comprising one or more washings of the precipitate with an aqueous solution comprising nitric acid, each washing being followed by a separation of the precipitate from the washing aqueous solution. 
     
     
         8 . The method of  claim 1 , further comprising one or more washings of the precipitate with an organic solution comprising the organic diluent, each washing being followed by a separation of the precipitate from the washing organic solution. 
     
     
         9 . The method of  claim 1 , wherein the actinide(IV) is plutonium(IV) or thorium(IV). 
     
     
         10 . The method of  claim 9 , wherein the actinide(IV) is plutonium(IV). 
     
     
         11 . A method for processing an aqueous solution issued from a dissolution of a spent nuclear fuel in nitric acid, the aqueous solution A1 comprising at least uranium(VI) and an actinide(IV), the method comprising at least the steps of:
 a) co-extracting the uranium(VI) and actinide(IV) from the aqueous solution A1, the co-extracting comprising at least one contact between the aqueous solution A1 and an organic solution comprising tri-n-butyl phosphate in an organic diluent, and then separating the aqueous solution from the organic solution, whereby the uranium(VI) and the actinide(IV) are present in the organic solution issued from step a) as uranium(VI) nitrate and actinide(IV) nitrate at concentrations such that a uranium(VI) nitrate concentration is higher than an actinide(IV) nitrate concentration, and a sum of the uranium(VI) nitrate concentration and the actinide(IV) nitrate concentration is greater than or equal to 55 g/L;   b) stripping the actinide(IV) and a fraction of the uranium(VI) from the organic solution issued from step a), the stripping comprising:
 at least one contact between the organic solution issued from step a) and an aqueous solution A2 comprising from 2 mol/L to 6 mol/L of nitric acid, and oxalic acid at a concentration greater than or equal to 18 g/L, with an organic solution/aqueous solution A2 volume ratio greater than or equal to 1, the oxalic acid concentration in the aqueous solution A2 and the organic solution/aqueous solution A2 volume ratio being selected so that the oxalic acid is deficient with respect to the stoichiometric conditions of a complete precipitation of uranium(VI) and actinide(IV), whereby a precipitate is obtained comprising the actinide(IV) in oxalate form and a fraction of the uranium(VI) in oxalate form with a U(VI)/actinide(IV) mass ratio of between 0.5 and 5; then 
 a separation of the precipitate from the organic solution and aqueous solution A2; and 
   c) stripping from the organic solution issued from step b) the uranium(VI) that was not stripped in step b), the stripping comprising at least one contact between the organic solution issued from step b) and an aqueous solution A3 comprising from 0.005 mol/L to 0.05 mol/L of nitric acid, and then a separation of the organic solution from the aqueous solution A3.   
     
     
         12 . The method of  claim 11 , wherein the organic solution comprises from 25% to 35% (v/v) of tri-n-butyl phosphate. 
     
     
         13 . The method of  claim 11 , wherein the oxalic acid concentration in the aqueous solution A1 is greater than or equal to 20 g/L. 
     
     
         14 . The method of  claim 13 , wherein the oxalic acid concentration in the aqueous solution A1 is greater than or equal to 22 g/L. 
     
     
         15 . The method of  claim 11 , wherein the organic solution/aqueous solution A1 volume ratio is greater than or equal to 1.5. 
     
     
         16 . The method of  claim 11 , further comprising, between steps a) and b), a washing of the organic solution issued from step a), the washing comprising at least one contact between the organic solution issued from step a) and an aqueous solution A4 comprising from 0.5 mol/L to 6 mol/L of nitric acid, and then a separation of the organic solution from the aqueous solution A4. 
     
     
         17 . The method of  claim 16 , wherein the aqueous solution A4 comprises from 4 mol/L to 6 mol/L of nitric acid. 
     
     
         18 . The method of  claim 11 , further comprising a regeneration of the organic solution issued from step c) for reuse thereof in step a). 
     
     
         19 . The method of  claim 11 , further comprising a conversion of the precipitate issued from step b) into a mixed uranium(VI) and actinide(IV) oxide. 
     
     
         20 . The method of  claim 11 , wherein the actinide(IV) is plutonium(IV) or thorium(IV).